FAN7621B Fairchild Semiconductor, FAN7621B Datasheet - Page 10

• Manufacturer Part Number: FAN7621B
• Description: The FAN7621B is a pulse frequency modulation controller for high-efficiency half-bridge resonant converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN7621B • Rev. 1.0.1
proportional to the switching frequency, the soft-start is
implemented by sweeping down the switching frequency
from an initial high frequency (f
voltage is established. The soft-start circuit is made by
connecting R-C series network on the R
in Figure 19. FAN7621B also has an internal soft-start
for 3ms to reduce the current overshoot during the initial
cycles, which adds 40kHz to the initial frequency of the
external soft-start circuit, as shown in Figure 20. The
initial frequency of the soft-start is given as:
It is typical to set the initial (soft-start) frequency of two ~
three times the resonant frequency (f
network.
The soft-start time is three to four times the RC time
constant. The RC time constant is as follows:
4. Control Pin: The FAN7621B has a control pin for
protection, cycle skipping, and remote on/off. Figure 21
shows the internal block diagram for control pin.
Protection: When the control pin voltage exceeds 5V,
protection
described in the protection section.
Pulse Skipping: FAN7621B stops switching when the
control pin voltage drops below 0.4V and resumes
switching when the control pin voltage rises above 0.6V.
To use pulse-skipping, the control pin should be
connected to the opto-coupler collector pin. The
frequency that causes pulse skipping is given as:
T
f
CON
SS
ISS
=
f
Figure 20. Frequency Sweeping of Soft-Start
=
ISS
R
(
f
6
SS
Figure 21. Internal Block of Control Pin
s
5.2
R
⋅
LV
LV
OLP
OVP
C
min
23V
k
5V
CC
CC
is
SS
Ω
I
0.4 / 0.6V
OLP
+
40kHz
triggered.
+
-
+
-
5.2
-
+
R
SS
k
Ω
) 100 40 (
×
LV
Detailed
CC
good
+
I S S
Control loop
kHz
take over
) until the output
O
Auto-restart
protection
applications
) of the resonant
R
S
T
)
-Q
pin, as shown
Q
Stop Switching
time
are
(3)
(4)
10
Remote On / Off: When an auxiliary power supply is
used for standby, the main power stage using
FAN7621B can be shut down by pulling down the control
pin voltage, as shown in Figure 23. R1 and C1 are used
to ensure soft-start when switching resumes.
5. Protection Circuits: The FAN7621B has several self-
protective functions, such as Overload Protection (OLP),
Over-Current Protection (OCP), Abnormal Over-Current
Protection (AOCP), Over-Voltage Protection (OVP), and
Thermal Shutdown (TSD). OLP, OCP, and OVP are
auto-restart mode protections; while AOCP and TSD are
latch-mode protections, as shown in Figure 24.
SKIP
Figure 22. Control Pin Configuration for Pulse
R
m ax
=
V
CC
Figure 23. Remote On / Off Circuit
5
R
2 .
R
min
m in
k
+
. 4
R
C
16
R
max
SS
SS
k
Skipping
x
CON
RT
CS
100
(
LV
kHz
SG
CC
HV
)
CTR
HO
LO
CC
www.fairchildsemi.com
PG
(5)